Friction brake and clutch devices generally employ relatively rotatable members having substantially solid, mutually facing friction surfaces which frictionally engage to absorb and/or transmit kinetic energy. One of the surfaces is typically defined by a so-called friction material. Relative rotation of the friction sufaces, while they are engaged, of course, converts the slip portion of the kinetic energy to heat. Friction brake devices are typically thought of as power absorbers and friction clutch devices as power transmitters. However, in many applications, friction clutch devices may have relatively long continuous slip duty cycles and, therefore, have to absorb great amounts of power in the form of heat. If prolonged or continuous slip duty cycles of brake or clutch devices are such that power absorption causes overheating, the devices are designed to dissipate the heat at a rate sufficient to prevent the damaging effects of overheating. Overheating is known to cause undesirable friction material performance including variations in the coefficient of friction such as temporary or permanent fade, accelerated wear and failure of the friction material, complete failure of the device, fire, etc.
The vast majority of brakes and clutches are used in applications requiring relatively short duty cycles. Most clutch engagements are characterized by high peak or instantaneous energy absorption and low total energy absorption per duty cycle. Clutch engagement duty cycles are typically a maximum of two seconds with high loading per square unit of friction material surface; for example, the peak energy rate often varies from 0.5 to 5 hp/in.sup.2 (57.8 to 578 watts/cm.sup.2) of friction surface; and total energy absorption is low due to the short duty cycle.
Most brake engagements are characterized by somewhat lower instantaneous energy absorption peaks but relatively higher total energy absorption per duty cycle. Brake engagement duty cycles are typically 2-30 seconds with moderate loading per square unit of friction material surface; for example, average energy rate often varies from 0.5 to 0.1 hp/in.sup.2 (57.8 to 11.6 watts/cm.sup.2) of friction surface; and total energy absorption is normally higher than for clutch engagements due to the longer duty cycles. Further, to reduce fade and other undesirable characteristics, such as excessive wear, an average energy rate of less than 0.5 hp/in.sup.2 (11.6 watts/cm.sup.2) is often recommended.
Clutches and brakes used in continuous slip applications, i.e., duty cycles typically much longer than 30 seconds, are characterized by low instantaneous energy absorption per duty cycle. Brakes and clutches rated for continous slip are, preferably, capable of 6 to 12 minute duty cycles. The average energy rate is typically less than 0.1 hp/in.sup.2 (11.6 watts/cm.sup.2) of friction surface; however, total energy absorption per square unit of friction surface may be 800 times greater than the total energy absorption of a typical clutch engagement or duty cycle.
The continuous presence of liquid on the friction surfaces in brake and clutch devices typically determines whether the devices are classified as dry or wet. Friction surfaces which run dry, typically in free air, are classified as dry friction devices. Friction surfaces running in a liquid or having a liquid flowing therebetween are typically classified as wet devices. The dry devices are often cooled only by air conduction and/or convection; however, such devices may be indirectly cooled by a liquid such as disclosed in U.S. Pat. No. 2,821,271 to Sanford and drawn to a motor vehicle brake cooled by an ethylene glycol/water solution taken from the vehicle radiator. Effectiveness of indirect cooling depends greatly on the thermal conductivity of the material between the friction surfaces and the liquid coolant. A friction material which is run dry typically wears several hundred times as fast as the same material run wet in a given application. Further, dry friction materials are substantially more susceptible to chatter/stick-slip and fade.
It is known that continuous slip power absorption, per square unit of a friction surface in brake and clutch devices, may be raised substantially by direct liquid cooling of the friction surfaces. Direct liquid cooling also greatly reduces the wear rate of the friction materials. The liquid is typically an oil as disclosed in U.S. Pat. No. 4,291,794 and as disclosed in applicant's published European Patent Application 0,037,104. Direct water cooling of a brake having a molded asbestos friction material is proposed in U.S. Pat. No. 2,887,961 to Hawley. However, such direct water cooling of friction materials has had little or no lasting success. It is noted that the water coolant of Hawley does not recirculate and that water cooling of a somewhat related friction material, as discussed hereinafter, resulted in failure of the friction material. Oils so thoroughly dominate friction material cooling , that actual applications using water are virtually unknown or rare. This is so, particularly, in brake and clutch applications requiring continuous slip operation, such as in retarder and tensioner applications, where the lubricity of the oil seems to provide a protective hydroviscous effect which tends to lessen variations or transitions in friction coefficients. Such variations or transitions are associated with the phenomenon of chatter/stick-slip which often becomes pronounced as the relative speed of the friction surfaces decrease.
Oils, however in spite of their great use, do have several disadvantages in many applications. Oils greatly decrease the coefficient of friction of the friction surfaces and, thereby, require higher actuation forces, an increased number of friction surfaces, or an increase in the mean radius of the friction surfaces to obtain a given torque transmission or power absorption. Oils have a lower specific heat capacity than water or water solutions, such as water/glycol solutions or water/oil solutions. Therefore, a given cooling capacity requires greater amounts of circulating oil. Similarly, it is well known that the heat transfer properties of oil in heat exchangers is substantially less than that of water. Hence, oil coolant systems require larger heat exchangers. Oils are more expensive than water and most water solutions, and many brake/clutch applications require a redundant or separate cooling system when the coolant is oil. Oils are typically flammable and therefore potentially hazardous in their applications, whereas water or water solutions, such as ethylene glycol/water or many water oil solutions are fire resistant. And last but not least, the hydroviscous effect of oil seems to break down as the relative speed of the friction surfaces gets very low or nears zero. At these relative speeds, the chatter/slick-slip phenomenon often becomes very pronounced and the coefficient of friction increases rapidly with resultant premature lockup and high lockup torque spikes.